Japanese Laid-Open Patent Publication No. 2008-157289 (referred to as Patent Document 1 below) discloses a technical idea of a rotary actuator of this type. An object of the invention disclosed in this Patent Document 1 is to realize a thin rotary actuator. However, according to a specific configuration of this invention, a pressure receiving surface that receives a fluid to actuate a piston has a substantially circular shape. Therefore, the rotary actuator cannot be formed thinly so much.